1. Field of the Invention
The present invention relates to an image processing device and an image processing method with a capability of perceptually reproducing an image in an equal hue, and further relates to a technique suitable for color converting devices such as a display, a scanner, a digital camera, a color printer, a color facsimile, a color hard copier and software used in the color printer that runs on a personal computer (PC) or a workstation.
2. Description of the Related Art
Generally, display devices such as a cathode ray tube (CRT) and a liquid crystal display (LCD) and a printing device such as a printer are widely used as devices for outputting color images. However, due to different output method of respective display device and the printing device, discrepancies occur in a reproducible color range. If the same image data is output by using different output devices, for example, if an image created on the LCD is to be printed using the printer, colors that cannot be reproduced are likely to be produced. Thus, while handling the color images on a plurality of devices, a technique, in other words, a color conversion process that is called a gamut processing needs to be carried out for mapping color image signals to a color reproducible by the output devices.
As disclosed in Japanese Patent Application Laid-open No. 2004-104777, in the existing gamut processing technique, while converting a first color signal of a device-independent color space into a second color signal of a device-independent color space belonging to a color gamut of a second device, the color gamut mapping is carried out by using a corrected device-independent color space. The corrected device-independent color space is corrected from a curved surface, of the equal hue, including an achromatic axis in the device-independent color space or a curved surface conforming to the equal hue to a half plane including the achromatic axis and then mapped.
As disclosed in Japanese Patent Application Laid-open No. 2002-152536, the color signals are converted into a first intermediate color signal of the device-independent color space in which hue distortion on an input side color space is removed. With respect to the first intermediate color signal, gamut mapping is carried out by hue correction, lightness compression, and color saturation compression and the first intermediate color signal is converted into a second intermediate color signal. The second intermediate color signal is converted into an output side color space such that equal hues in the device-independent color space become linear in the output side color space.
Furthermore, as disclosed in Japanese Patent No. 3337697, based on the color saturation, a non-linear line that causes the hue in a uniform perceptual color space to change is prepared for a plurality of different hues in the uniform perceptual color space. Furthermore, by using the non-linear line corresponding to the hue in the uniform perceptual color space, an input color data is converted within a color reproducing range of a color image output device.
As disclosed in Japanese Patent Application Laid-open No. 2002-262120, a maximally saturated color in the hue that is matched with an input color signal Pi is calculated as a standard color signal T and by referring to a gamut data of the output device, a corresponding color signal M that corresponds to the standard color signal T, and that is having a minimum color difference within the color reproducing range of the output device is calculated. An output hue deciding unit decides, based, on the corresponding color signal M, a hue h of an output color signal Po and a mapping processor calculates lightness and saturation components. The lightness and saturation components are output as the output color signal Po, thus reducing color change at the time of mapping.
If a device-dependent color space is assumed as a standard red, green, blue (sRGB), the device-independent color space is assumed as color signals of a color appearance model international commission on illumination color appearance model 2002 (CIECAM02) (for CIECAM02, see a technical report CIE 159:2004 of CIE), and input RGB color signals are converted into the color signals of CIECAM02, a magnitude relation between hue angles becomes reverse.
In other words, as shown in FIG. 11, RGB values on two outermost contours of an sRGB color space are (R, G, B)=(0, 64, 255) and (R, G, B)=(0, 0, 255). In a hue, lightness, saturation (HLS) space, (R, G, B)=(0, 64, 255) corresponds to (H, L, S)=(225, 0.5, 1.0) and (R, G, B)=(0, 0, 255) corresponds to (H, L, S)=(240, 0.5, 1.0). In other words, in the HLS color space, although hue angles H are respectively H1=225 degrees, and H2=240 degrees (H1 is less than H2), if the HLS color space is converted into the color space of CIECAM02, a hue angle h corresponding to the hue angle H1=225 degrees approximately becomes h1=258.6 degrees and the hue angle h corresponding to the hue angle H2=240 degrees approximately becomes h2=257.9 degrees (h1 is greater than h2). The magnitude relation between the hue angles becomes reverse.
As indicated in expression (1) that is shown in FIG. 14, the HLS is a space that can be mutually converted with RGB in a linear shape. The HLS is a color space in which the RGB space is indicated by H (hue), L (lightness), and S (saturation).
Among jacbc color signals of CIECAM02, a horizontal axis and a vertical axis shown in FIG. 11 are respectively indicated by ac and bc that indicate chromaticity. As shown in FIG. 12, JCh color signals that indicate the color space of CIECAM02 by lightness, saturation, and a hue angle only differ with respect to whether the JCh color signals are in an orthogonal coordinate system or a cylindrical coordinate system. The hue angles h of some color signals are angles that are in a counterclockwise direction between the ac axis plus direction and some color signals. The plus direction of the ac axis is assumed as 0 degrees.
Thus, the magnitude relation between the hue angles becomes reverse when the device-dependent sRGB color signals are converted into the color signals of CIECAM02. However, reversing of the magnitude relation between the hue angles is not taken into consideration in Japanese Patent Application Laid-open No. 2004-104777. Thus, if the color is reproduced by the second device after color gamut mapping, the color is reproduced even though the magnitude relation between the hue angles is in a reverse state. Furthermore, the mapping is carried out by using the corrected device-independent color space that is corrected from the curved surface, of equal hues, including the achromatic axis in the device-independent color space or the curved surface conforming to the equal hues to the half plane including the achromatic axis and mapped. However, the color of the achromatic axis of the device-dependent color space changes (the achromatic axis of sRGB and the achromatic axis of Jacbc space do not completely match).
Similarly, in Japanese Patent Application Laid-open No. 2002-152536 and Japanese Patent No. 3337697, reversing of the magnitude relation between the hue angles while converting the device-dependent color signals on the input side into the color signals of the device-independent color space is not taken into consideration.
As disclosed in Japanese Patent Application Laid-open No. 2002-262120, the color gamut mapping can be carried out such that the color change is not prominently visible and input signals of the same hue always become output signals of the same hue. However, as shown in FIG. 13, based on the color space of the input signals, from a maximum saturation point of blue hue, the hue significantly approaches towards purple color on a white point side. If the color is reproduced without changing the hue of the input color signals, although the color reproduced is loyal to the hue of the input signals, isochromaticity is deteriorated (FIG. 13 is a schematic diagram in which the blue hue (from the white point to the maximum saturation of blue and further to a black point) of sRGB is converted into a Jacbc value of CIECAM02 and is plotted in an acbc plane. An equal hue line in the sRGB color space from the white point to a maximum saturation point is significantly curved on the plus direction of ac.